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Hunting for Brown Dwarf Planets

Brown dwarfs fascinate me because they’re the newest addition to the celestial menagerie, exotic objects about which we know all too little. The evidence suggests that brown dwarfs can form planets, but so far we’ve found only a few. Two gravitational microlensing detections on low mass stars have been reported, one of which is a 3.2 Earth-mass object orbiting a primary with mass of 0.084 that of the Sun, putting it into the territory between brown dwarfs and stars. The MEarth project has uncovered a planet 6.6 times the mass of the Earth orbiting a 0.16 solar mass star.

Now a new proposal to use the Spitzer Space Telescope to hunt for brown dwarfs planets is available on the Net, one that digs into what we’ve found so far, with reference to the discoveries I just mentioned:

Accounting for their low probabilities, such detections indicate the presence of a large, mostly untapped, population of low mass planets around very low mass stars (see also Dressing & Charbonneau (2013)). Arguably the most compelling discovery is that of the Kepler Object of Interest 961, a 0.13 [solar mass] star, orbited by a 0.7, a 0.8 and a 0.6 Rearth on periods shorter than two days (Muirhead et al. 2012). The KOI-961 system, remarkably, appears like a scaled-up version of the Jovian satellite system. This is precisely what we are looking for.

The plan is to use the Spitzer instrument to discover rocky planets orbiting nearby brown dwarfs, the idea being that the upcoming mission of the James Webb Space Telescope will need a suitable target list, and soon, for it to be put to work on probing the atmospheres of exoplanets. A 5400 hour campaign is the objective, the goal being to detect a small number of planetary systems with planets as small as Mars. Interestingly, the team is advocating a rapid release of all survey data to up the pace of exoplanet research and compile a database for further brown dwarf studies.


Image: The stellar menagerie: Sun to Jupiter, via brown dwarfs. Credit: Space Telescope Science Institute.

Brown dwarfs turn out to be excellent targets as we try to learn more about rocky planets around other stars. Studying the photons emitted by an atmosphere during an occultation requires relatively close targets, and as the paper on this work points out, the fainter the primary, the better the contrast between the central object and the planet. And around brown dwarfs we can expect deep transits that allow us to detect objects down to Mars size with Spitzer’s equipment. The paper also notes that brown dwarfs older than half a billion years show a near constant radius over their mass range, making it easier to estimate the size of detected planets.

Spitzer is the only facility that can survey a sufficient number of brown dwarfs, long enough, with the precision and the stability required to credibly be able to detect rocky planets down to the size of Mars, in time for JWST. We estimate that about 8 months of observations would be needed to complete the survey. Once candidates are detected, large ground-based facilities will confirm the transits, find the period (if only one event was captured by Spitzer) and check for the presence of additional companions. This program will rapidly advance the search for potentially habitable planets in the solar neighborhood and transmit to JWST a handful of characterizable rocky planet atmospheres.

This is a survey that not only probes a fascinating kind of object but one that should offer what the paper calls “the fastest and most convenient route to the detection and to the study of the atmospheres of terrestrial extrasolar planets.” It goes public at a time when 76 new brown dwarfs have been discovered by the UKIRT Infrared Deep Sky Survey, including two potentially useful ‘benchmark’ systems. The authors of the Spitzer proposal argue that observing the atmospheres of Earth-sized transiting worlds around M-Dwarfs with JWST will be much more challenging than equivalent work using brown dwarfs, assuming we get to work identifying the best targets.

The white paper is Triaud et al., “A search for rocky planets transiting brown dwarfs,” available online. The UKIRT Infrared Deep Sky Survey paper is Burningham et al., “Seventy six T dwarfs from the UKIDSS LAS: benchmarks, kinematics and an updated space density,” accepted at Monthly Notices of the Royal Astronomical Society (abstract).


Comments on this entry are closed.

  • Adam April 30, 2013, 16:37

    If we find a brown dwarf closer than Proxima Centauri, I wonder how tempting it will be? Objects with shrinking habitable zones may be less attractive than stars, but one thing that the observed bestiary of odd-ball planets should have taught us, is that Nature doesn’t conform to our prejudices.

  • coolstar April 30, 2013, 16:55

    Interesting. I sorta doubt they’ll get that much time though, especially since the most well known authors are well down the author list (oh yes, indeed, that does matter). Since brown dwarfs are quite rare compared to M dwarfs, I’m not sure I follow the logic behind the proposal, especially since there are likely to be few rocky planets in the habitable zones (VERY close in and rapidly evolving) around them.

  • Harry R Ray May 1, 2013, 9:21

    The big problem with brown dwarf habitable zones is that they very greatly over time. However.,recent microlenzing studies sugtgest a way around this! Very close BINARY brown dwarfs are now known to exist, meaning that CONTACT BINARY brown dwarfs are possible! A very close orbiting brown dwarf system’s habitable zone during its youth may be maintained when the two brown dwarfs cool down if they come into contact with each other. A harsh radiation environment may ensue, but a planet’s magnetic field may be able to protect it.

  • coolstar May 1, 2013, 19:38

    @HRH contact binary brown dwarfs must be quite rare since those are the easiest types of eclipsing binaries to find. Your point about the magnetic fields is very interesting as SOME brown dwarfs are known to have very strong magnetic fields (as well as to flare), akin to that of a super-charged Jupiter. The ratio of brown dwarfs to red dwarfs is roughly 1:4 but red dwarfs are typically a factor of roughly 1000 times brighter……thus for transit studies of atmospheres, they make immensely better “flashlights”. Contrast is not EVERYTHING when studying planetary systems as the sheer number of photons available tends to be important also. Any rocky planet orbiting in the habitable zone of a BD is likely to be very entangled in the mutual magnetic fields. I suspect for these and other reasons the atmospheres of brown dwarf planets are going to be anything but typical of those of rocky planets in general. Again, very interesting, but perhaps not terribly relevant for the study of life in the universe.

  • andy May 6, 2013, 15:57

    IIRC the latest-type contact binaries are somewhere around mid-K. I seem to recall that this may be to do with the increasing amount of convection in low-mass stars making the systems far more unstable to tidal decay, but I could well be talking nonsense here. Brown dwarfs don’t seem to be good candidates for contact binaries, unlike stars which expand as they age (which probably helps in bringing about the common envelope), brown dwarfs shrink.

  • ljk July 29, 2013, 11:57


    The Physics arXiv Blog

    July 29, 2013

    First Planet Discovered Orbiting a Brown Dwarf

    Astronomers have long supposed that planets can form around brown dwarfs just as they do around ordinary stars. Now they’ve found the first example.

    Astrophysical calculations show that any star that is smaller than about 1/10th of the mass of the sun cannot sustain hydrogen fusion reactions at its core. These failed stars never light up. Instead they wander the galaxy as warm, dark balls of hydrogen known as brown dwarfs.

    Brown dwarfs probably form through the same process that lead to ordinary stars but merely on a smaller scale. If that’s correct, planets should also form in the protoplanetary disks of gas and dust around brown dwarfs. Indeed, astronomers have seen a number of protoplanetary disks of this type.

    Until now, however, they’ve never seen a planet orbiting a brown dwarf. That’s not really surprising.

    The standard methods for detecting planets look for the way a star wobbles as a planet orbits or at how its magnitude changes as a planet passes in front. But given that brown dwarfs are dim and difficult to see, these methods have yet to produce fruit.

    All that changes today with the announcement by an international team of astronomers that they’ve discovered a planet orbiting a brown dwarf the first time. These guys have made their discovery using an entirely different method of detection called gravitational lensing. This occurs when one body passes in front of another and its gravity focuses light from the more distant object towards Earth. That works regardless of the brightnesses involved.

    The brown dwarf in question is almost 6000 light years from Earth in the Fish Hook constellation. Astronomers first noticed an unusual change in its brightness in April 2012. Further investigation showed that this was indeed a lensing event.

    These guys conclude that the brown dwarf is being orbited by a planet about twice the mass of Jupiter at a distance of just under one astronomical unit. The brown dwarf itself is about 10 times larger than its companion.

    That’s the first time astronomers have found an object orbiting a brown dwarf that can be truly described as a planet. The technical definition of a planet is that it must have formed in the parent object’s protoplanetary disk.

    Astronomers have seen other planet-sized objects orbiting brown dwarfs but only at distances of several tens of astronomical units. That’s too far to have been part of the protoplanetary disk. “Thus,…,they are not bona fide planets,” say the team.

    So that’s a modest first for this team. It raises the question of what kind of conditions exist on such a planet and, of course, whether these could support life.

    This planet almost certainly does not fall into that category but where there is one planet, there are almost certainly others. Astronomers can now have some fun speculating on the Goldilocks zones around brown dwarfs where conditions are just right for life and how to spot the interesting planets inside them.

    Ref: http://arxiv.org/abs/1307.6335 : Microlensing Discovery Of A Tight, Low Mass-Ratio Planetary-Mass Object Around An Old, Field Brown Dwarf